The present invention is directed to systems and methods for adaptive equalization of signals in communication systems, and more particularly to a system and method in which a received signal is evaluated without reference to an input signal to determine whether nonlinearities are present and to provide information for correcting the nonlinearities.
Nonlinearities in communication system components can distort a communicated signal so that the signal's bit error rate (BER) may be unacceptably degraded. For example, power amplifiers have nonlinear amplitude modulation to amplitude modulation (AM/AM) and amplitude modulation to phase modulation (AM/PM) conversion characteristics which can distort signals passed therethrough. This problem and several potential solutions are discussed in U.S. Pat. No. 4,291,277 that is incorporated by reference.
One method of correcting nonlinearities in a signal is to predistort the signal before it is processed for transmission in order to compensate for nonlinearities that are introduced during processing. A predistortion correction is typically determined by comparing an input signal to an output signal, and feeding back the determined differences to correct the input signal. The correction of the nonlinearities may be adaptive so that the corrections change as the nonlinearities change, such as discussed in the above referenced patent.
However, such methods typically require that a copy of the input signal be preserved for comparison to the output signal and typically include a shift register or other delay mechanism for this purpose. Other systems may use a pilot tone as an ersatz for the input signal. It would be desirable to avoid the comparison of the input signal to the output signal and eliminate this delay, and to avoid the use of pilot tones which can decrease available bandwidth.
The present invention relies on characteristics of the received signal that indicate when nonlinearities are present, without resorting to comparison to either the input signal or a pilot tone and without the delays attendant therewith. Thus, with the present invention nonlinearities may be determined at any place in the system without reference to an input or pilot signal.
Accordingly, it is an object of the present invention to provide a novel method and system for compensating for nonlinearities in a communication system that avoids comparisons to other signals and thereby obviates the problems of the prior art.
It is another object of the present invention to provide a novel method and system for compensating for nonlinearities introduced when a signal is transmitted in which characteristics of a transmitted signal are evaluated to determine whether the signal includes nonlinearities.
It is yet another object of the present invention to provide a novel method and system for compensating for nonlinearities introduced when a signal is transmitted in which characteristics of a signal are evaluated at a receiver, without resort to the originally transmitted signal or pilot tone, and the results of the evaluation are sent back to the transmitter for predistortion of signals transmitted therefrom.
It is still another object of the present invention to provide a novel method and system for predistorting an RF signal to compensate for nonlinearities introduced when the RF signal is communicated on a first frequency in which nonlinearity corrections are generated at a receiver and provided to a collocated transmitter through a reverse channel connection for transmission back to the original transmitter on a second frequency.
It is a further object of the present invention to provide a novel method and system for predistorting an RF signal to compensate for nonlinearities introduced when the RF signal is communicated in which nonlinearity corrections are provided back to the original transmitter that includes a predistorter connected to a collocated receiver through a reverse channel connection.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.